Finely divided oxide powders are useful in the manufacture of coating compositions, intricately shaped and fine-grained ceramics, cermets, and the like. Small particles are particularly important in the preparation of powder mixtures. In general, the smaller the particle size, the more uniform are the compositions and the better the mechanical properties of metal, ceramic, and cermet articles prepared from the powder mixtures. There is further advantage if the fine-grain particles have equiaxed dimensions, i.e., the particles are substantially microspheric in shape.
A variety of techniques have been attempted for the production of inorganic powders that are characterized by particles which are fine grained and microspheric in dimensions. Grinding of inorganic powders can result in very fine particles, but conchoidal fracture and other cleavage effects during grinding yield particles with sharp edges and jagged contours.
Shock cooling, spray drying, and prilling also have been investigated for this purpose. In most cases these methods provide powder particles that have one or more undesirable physical properties such as porosity, nonuniformity of particle size and shape, lack of crystal homogeneity, and the like.
A recent development has been the study of sonication as a prospective energy source relative to physicochemical phenomena.
As described in publications such as J. Appl. Phys., 37, 254(1968), it has been recognized that nucleation of a crystal in an undercooled liquid can be induced by cavitation. The cavities are small voids in the liquid which open in negative pressure regions, as can be produced by ultrasonic wave energy. High local pressures result when the cavities collapse. It appears that most forms of mechanical disturbance nucleate by creating cavities which collapse to produce the nuclei. It has been observed that an ultrasonic field which produces cavitation in the vicinity of growing crystals can cause extensive fracturing of the crystals. Ultrasonic irradiation of castings has been utilized to achieve grain refinement.
Ukr. Khim. Zh., 43(12), 1285(1977) describes a study involving the use of an ultrasonic field as an analytical tool to monitor the gelling of a sodium silicate solution.
Chemical Week (page 29, Jan. 18, 1984) reports the growing interest in the potential utilization of sonication in chemical processes, such as palladium-catalyzed alkyne and alkene hydrogenation, Friedel-Crafts acylation of aromatic compounds to produce ketones, reduction of nitro compounds to anilines, zinc-catalyzed condensation of carbonyl compounds and bromoesters to beta-hydroxyesters, and the like.
Prior art of more specific interest with respect to the present invention are various processes disclosed in the United States patent literature.
U.S. Pat. No. 3,222,231 describes a process which involves the physical precipitation of a solid solute (e.g., ammonium perchlorate) out of solution while simultaneously subjecting the solution to both mechanical agitation and high frequency vibration.
U.S. Pat. No. 3,249,453 describes a process for producing finely-dispersed dyestuffs and pigments which involves initially forming particles of a dyestuff or pigment in statu nascendi (i e., by precipitation from a solution) in an ultrasonic field having at the locus of formation a maximum of sound intensity, and thereafter exposing the color component to a zone of decreasing sound intensity.
U.S. Pat. No. 3,408,304 describes a process for preparing an oxide nuclear fuel material which involves the steps of forming a slurry of actinide precipitate, and then converting the slurry of actinide precipitate to a sol by peptizing the slurry with the aid of ultrasonic wave energy.
U.S. Pat. No. 3,617,584 describes a process for preparing spherical particles from an aqueous feed of dispersed metal oxide such as uranium oxide or plutonium oxide, which involves subjecting the aqueous feed to sonic vibrations of predetermined frequency and amplitude.
There is continuing interest in the prospective advantages of new and improved applications of sonication to physicochemical transformations.
Accordingly, it is an object of this invention to provide an improved process for production of microcrystalline inorganic oxide powders.
It is another object of this invention to provide a process which employs ultrasonic wave energy for the production of fine grain metastable tetragonal zirconia powder which is free of monoclinic and cubic phase crystal structures.
It is another object of this invention to provide a process which employs ultrasonic wave energy for the production of a fine grain ferrimagnetic spinel composition having a ferrite crystal lattice structure of improved dimensional stability and strength.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.